1. Field of the Invention
This invention relates to electrical switching apparatus and, more particularly, to circuit breakers, such as, for example, aircraft circuit breakers.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. As such, they are subjected to heavy use and, therefore, must be capable of performing reliably over many operating cycles. They also must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
The circuit breaker push-pull handle is moved from in-to-out in order to open the load circuit. This action may be either manual or, else, automatic in the event of an overload or fault condition. If the push-pull handle is moved from out-to-in, then the load circuit is re-energized. If the load circuit had been automatically de-energized, then the out-to-in operation of the push-pull handle corresponds to a circuit breaker reset action.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection. Arc faults are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire.
Although many circuit breakers also employ ground fault protection, in aircraft applications, the aircraft frame is ground, and there is no neutral conductor. Some aircraft systems have also provided ground fault protection, but through the use of additional devices, namely current transformers which in some cases are remotely located from the protective relay.
During sporadic arcing fault conditions, the overload capability of the circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements required for sputtering arc fault protectionxe2x80x94ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. It is still desirable, however, to provide separate indications in order to distinguish an arc fault trip from an overcurrent-induced trip.
Finally, there is an interest in providing an instantaneous trip in response to very high overcurrents such as would be drawn by a short circuit.
The challenge is to provide alternative protection and separate indications in a very small package, which will operate reliably with heavy use over a prolonged period. A device which meets all the above criteria and can be automatically assembled is desirable.
In aircraft applications, two practical considerations make automatic operation difficult to achieve and, possibly, undesirable. First, the design of a conventional aircraft circuit breaker makes it difficult to add an externally initiated tripping circuit thereto. Second, certain circuits on an aircraft are so critical that manual intervention by a crewmember may be desirable before a circuit is de-energized.
It is known to employ a conventional U-shaped bonnet around an arc chamber of a circuit breaker.
There is room for improvement in circuit breakers.
According to one aspect of the invention, a circuit breaker comprises: a housing; separable contacts mounted in the housing; an operating mechanism for opening and closing the separable contacts; an overcurrent assembly responsive to selected conditions of current flowing through the separable contacts for actuating the operating mechanism to trip open the separable contacts; and a bonnet having first and second pieces, the first piece forming a first leg of the bonnet, the second piece forming a second leg and a base of the bonnet, in order to form a U-shape which surrounds the separable contacts and which cools and splits an arc when the operating mechanism trips open the separable contacts.
As another aspect of the invention, a circuit breaker comprises: a housing; a pair of separable contacts mounted in the housing; an operating mechanism for opening and closing the separable contacts; a first terminal electrically interconnected with a first one of the separable contacts; a second terminal electrically connected to a second one of the separable contacts; an electrically conductive support mechanism mounted in the housing; and a bimetal overcurrent assembly responsive to selected conditions of current flowing through the separable contacts for actuating the operating mechanism to trip open the separable contacts, the bimetal overcurrent assembly having first and second legs and a free intermediate section which deflects in response to the selected conditions of current to actuate the operating mechanism, with the first leg engaging and being electrically connected to the support mechanism, with the second leg electrically connected to the first terminal, and with the support mechanism electrically interconnected with the first one of the separable contacts.
As a further aspect of the invention, a circuit breaker comprises: a housing having an opening therein; separable contacts mounted in the housing; a latchable operating mechanism comprising: a toggle mechanism having first and second pivotally connected toggle links coupled to the separable contacts for opening and closing the separable contacts, an operating handle assembly coupled to the toggle mechanism, the handle assembly including first and second pieces, with the first piece secured to the second piece, the first piece providing a first visual impression and the second piece providing a different second visual impression, and a latch assembly latching the toggle mechanism in a latched condition in which the toggle mechanism is manually operable by the handle assembly between a toggle open position and a toggle closed position to open and close the separable contacts, the latch assembly including a latch member which when released unlatches the toggle mechanism to open the separable contacts; and an overcurrent assembly responsive to selected conditions of current flowing through the separable contacts for releasing the latch member to trip the separable contacts open, wherein the first piece of the handle assembly is internal to the housing when the separable contacts are closed, wherein the second piece of the handle assembly is external to the housing, and wherein a portion of the first piece of the handle assembly is external to the housing when the separable contacts are open.
As another aspect of the invention, a circuit breaker comprises: a housing including a molded case, a molded cover and an external clip plate securing the molded cover to the molded case; separable contacts mounted in the housing; an operating mechanism for opening and closing the separable contacts; and an overcurrent assembly responsive to selected conditions of current flowing through the separable contacts for actuating the operating mechanism to trip open the separable contacts.
Preferably, the clip plate includes a top and two sides disposed therefrom, with one of the sides capturing the molded case and the other of the sides capturing the molded cover.